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AP Computer Science Principles · Unit 4 of 5
Computer Systems & Networks
11–15% of the exam4 lessons · 51 min28 terms
What this unit covers
The topics below follow the published CS Principles course framework for Unit 4. This unit is worth 11–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
The internetFault toleranceParallel computingProtocols
Lessons in this unit
- The Internet: Packets, Routing & Addresses14 min · 3 objectivesDescribe how data is divided into packets and reassembled · Explain how routing moves packets across many possible paths · Explain the role of IP addresses and the Domain Name System
- Fault Tolerance & Redundancy12 min · 3 objectivesDefine fault tolerance and explain why the Internet needs it · Explain how redundancy provides alternate paths and copies · Relate redundant routing to the Internet’s reliability
- Parallel & Distributed Computing13 min · 3 objectivesDistinguish sequential, parallel, and distributed computing · Compute the speedup of a parallel solution over a sequential one · Explain why speedup is limited by the portion that must run sequentially
- Protocols & Open Standards12 min · 3 objectivesDefine a protocol and explain why shared protocols are essential · Describe the roles of IP, TCP, and HTTP at a high level · Explain how open standards enable scalability and interoperability
Formulas in Unit 4
Path of a web request
name (example.com) → DNS lookup → IP address → packets routed hop-by-hop → reassembled at destination
DNS turns a name into an IP address; the message is split into packets that are routed independently and reassembled at the other end.
Fault tolerance through redundancy
more redundant paths/copies → fewer single points of failure → higher fault tolerance
Redundancy means having backups (extra connections, extra copies) so that when one part fails, the system continues using another.
Speedup
speedup = sequential time / parallel time
A speedup of 4 means the parallel solution runs in one-quarter the time. Total parallel time = time of the sequential-only portion + (parallel portion ÷ number of processors), roughly.
Protocol layers, at a glance
IP = addressing/routing · TCP = reliable ordered delivery · HTTP = web requests
Lower layers handle getting bits to the right place; higher layers build services (like the Web) on top of that foundation.
Every term in Unit 4
All 28 terms we publish for Computer Systems & Networks, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Computing device
- A machine that runs a program — a computer, phone, sensor, router, or embedded controller. Broader than "computer".
- Computing system
- A group of computing devices and programs working together toward a common purpose.
- Computer network
- Two or more computing devices connected so they can share data. The Internet is a network of networks.
- The Internet
- A global network of interconnected networks using agreed protocols. It is infrastructure — distinct from the World Wide Web, which is one service running on it.
- World Wide Web vs the Internet
- The Web is a system of linked documents and applications accessed over the Internet using HTTP. The Internet also carries email, streaming and much else.
- Protocol
- An agreed set of rules for how devices communicate. Without shared protocols, connected devices cannot exchange anything meaningful.
- IP
- Internet Protocol — assigns addresses to devices and routes packets between them. The addressing layer.
- TCP
- Transmission Control Protocol — splits data into packets, confirms delivery, requests missing packets, and reassembles them in order. Reliability comes from TCP, not from IP.
- UDP
- A protocol that sends packets without confirming delivery. Faster and lower-overhead than TCP, and used where a lost packet matters less than a delay — live video, games.
- HTTP
- The protocol for requesting and delivering web pages. Runs on top of TCP/IP.
- DNS
- The Domain Name System — translates human-readable domain names into IP addresses. The reason you can type a name instead of a number.
- Packet
- A small chunk of data with a header giving its source, destination and position in the sequence. Large messages are split into many packets.
- Packet switching
- Sending packets independently over whatever route is available, then reassembling them at the destination. Packets from one message can take different paths and arrive out of order.
- Why packet switching is fault tolerant
- No single fixed path is required. If one route fails, packets travel another way, and the message still arrives.
- Routing
- Choosing a path for a packet across the network. Routers make that decision hop by hop rather than a path being fixed in advance.
- Bandwidth
- The maximum amount of data that can be sent in a fixed time, usually in bits per second. A capacity, not a speed of travel.
- Latency vs bandwidth
- Latency is the delay before data begins arriving; bandwidth is how much arrives per second. A high-bandwidth link can still feel slow if latency is high.
- Fault tolerance
- A system's ability to keep working when part of it fails. Achieved through redundancy — more than one path or copy.
- Redundancy
- Duplicate paths, devices or data so that a failure has an alternative. The mechanism behind fault tolerance.
- Why the Internet is fault tolerant
- Redundant routing paths plus packet switching. Removing one connection rarely disconnects anything, because packets route around it.
- Scalability of the Internet
- New devices and networks can join without redesigning the whole system, because the protocols are open and addressing is hierarchical.
- Open protocol
- A protocol whose specification is public so anyone can implement it. The reason devices from different manufacturers interoperate at all.
- IPv4 vs IPv6
- IPv4 uses 32-bit addresses, giving about 4 billion; IPv6 uses 128-bit addresses, giving vastly more. The move to IPv6 exists because 4 billion ran out.
- Bit rate
- The number of bits sent per second. Transfer time ≈ file size in bits divided by bit rate.
- Client and server
- A client requests a service; a server provides it. The same machine can be both, for different transactions.
- Why a network diagram question is about paths
- These questions ask whether devices can still communicate after connections are removed. Trace the remaining paths rather than counting connections.
- Parallel computing on a network
- Distributed systems split a problem across many networked machines. Communication cost between them is what limits the gain.
- Sequential vs parallel solution time
- Compute the sequential total, then divide the parallelizable portion among processors and add back what must stay sequential. That sum is the answer.
What examiners penalize here
- Keep the roles distinct: **IP addresses** identify devices, **DNS** turns names into IP addresses, and **routing** picks each packet’s path. The exam mixes these up in answer choices on purpose.
- Link the concepts in a chain: **redundancy → multiple paths → no single point of failure → fault tolerance**. If an exam scenario has only one path or one copy, its weakness is the single point of failure.
- The exam’s key insight about parallelism: only the **parallelizable portion** speeds up, so the sequential part limits the total speedup. More processors give diminishing returns — never assume time falls in direct proportion to processor count.
- Connect **open standards** to **scalability** and **interoperability**: because the rules are public and shared, the Internet can keep growing and any device can interconnect. This cause-and-effect is a recurring exam theme.
Practice CS Principles
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Computer Science Principles exam is Unit 4?
Unit 4, Computer Systems & Networks, is worth 11–15% of the CS Principles multiple-choice section according to the published course framework. Across all 5 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in CS Principles Unit 4?
Computer Systems & Networks covers The internet, Fault tolerance, Parallel computing and Protocols. We publish 28 terms with definitions for this unit, all of them on this page.
How should I study CS Principles Unit 4?
Read the 4 lessons below first — about 50 minutes — then drill the 28 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 5 units of AP Computer Science Principles
Unit names, topics and exam weights follow the published College Board course framework for AP Computer Science Principles. AP® is a trademark registered by the College Board, which does not endorse this site.